As a conventional swivel joint, there is a swivel joint 30 shown in FIGS. 1 and 2 in Japanese Patent Laid-Open No. 10-332065. The swivel joint 30 includes three members of an inner cylinder 31, a middle cylinder 32, and an outer cylinder 33, hydraulic oil from a hydraulic pump is supplied through an inner cylinder oil passage 311, a middle cylinder oil passage 321, and an outer cylinder oil passage 331 formed in the cylinders 31, 32 and 33, respectively, to each hydraulic actuator, or drain oil from each actuator flows into a tank.
Specifically, the drain oil from the actuator flows through a body section 331a of the outer cylinder oil passage 331 into an outer annular groove 331c, then flows through a communication hole 321c in the middle cylinder oil passage 321 into an inner annular groove 321d, further flows through a communication hole 311c into a body section 311a of the inner cylinder oil passage 311, and finally fed to the tank through a drain oil passage connected to a connecting portion 311b. 
In the inner annular groove 321d and the outer annular groove 331c, annular packings (O-rings) 34 and 35 are provided having slide surfaces on an outer peripheral surface of the inner cylinder or an outer peripheral surface of the middle cylinder to prevent communication between the annular grooves and leakage of the drain oil.
However, the drain oil flowing through the annular grooves directly comes into contact with the slide surfaces and the annular packings, and dust, impurity, or abrasive powder in the oil bites into the slide surfaces or wear of the packings is accelerated, thereby reducing life of the swivel joint itself.
Also, in the conventional swivel joint, the swivel joint and the working device are connected by fastening flanges thereof by a bolt, and when the packing or the like is damaged and needs replacement, it is necessary that the bolt is removed to remove the working device and then remove the swivel joint itself, and further the swivel joint is taken apart for replacement of a component such as the packing, which takes much labor.